Looks cool, keeps the bolt from bashing into your mask, that can get annoying after a while, adds a little protection to your gun

Weaknesses:

My cousin got one for his gun and his gun was leaking air so he held the bolt and pulled the trigger to let the bolt in slower, the bolt slipped and he nearly lost his finger between the sight rail and bolt, so be careful.

Conclusion:

The beaver tail eleminates the problem of the bolt smacking your face but also creates the other problem, i give it a 6, i reccomend if you have a piranha with a sight rail, turn the sight rail around, it does the same thing as the beaver tail for nothing, and still looks cool